1. Field of the Invention
The present invention relates to an electrostatic latent image developing toner used in a developer when developing an electrostatic latent image formed by a method such as xerography and electrostatic recording. The present invention also relates to a method for manufacturing an electrostatic latent image developer and electrostatic latent image developing toner.
2. Description of the Related Art
In xerography, an electrostatic latent image is formed on a photoreceptor by performing a charging process and an exposure process. The electrostatic latent image is subsequently visualized by performing a developing process using a developer including a toner, a transfer process, and a fixation process. The developer may be a two-component developer composed of a toner and a carrier, or a single-component developer in which a magnetic or non-magnetic toner is used alone. Toners are commonly fabricated using a kneading-and-grinding method in which a thermoplastic resin is melted and kneaded with a pigment, a charge control agent, and a release agent such as wax. The mixture is then cooled, finely ground, and classified.
When such a conventional method is used, the shapes of toner particles become non-uniform. Further, the surface structures of the toner particles vary depending on the grindability of materials used and the conditions of the grinding process. As such, it is very difficult to control the shapes and surface structures of toner particles as desired.
Recently, wet fabrication methods for manufacturing a xerographic toner have been proposed as methods which allow control of the shape and surface structure of toner particles as desired. Among such wet fabrication methods are a wet spherification method which permits shape control, a suspension granulation method which permits surface composition control, a suspension polymerization method which permits internal composition control, and an emulsion polymerization aggregation method.
By chemically forming the toner particles using these methods, toners having very small particle sizes such as a volume average particle size of 5 μm, which were practically impossible to produce using a conventional method, are now commercially available at low cost. Further, according to the conventional method, because the shapes of the toner particles are non-uniform, resulting in a wide distribution of particle sizes and therefore requiring an increased number of toner particles per unit area, charge control of small-particle toners produced by the conventional method is difficult to be performed. However, by using the wet fabrication method, the particle sizes and shapes can be made uniform, and charge control is facilitated. For these reasons, further enhancements in production of high-quality images are desired, and it is widely expected that small-particle toners produced by the wet fabrication method will realize these enhancements.
In xerography, there exist two different methods for preventing adhesion of toner onto a fixation roller used when fixing toner on a sheet such as paper. One method is to apply oil on a heat roll when performing the fixation, so as to provide release property between the heat roll and the image being fixed. According to this method, no release agent is included in the toner particles. The second method is an oil-less method, in which a release agent such as wax is included within the toner particles. During the fixation process, the release agent within the toner particles are made to migrate to the image surface by the heat of the heat roll, thereby achieving release property. The second oil-less method is advantageous over the first method because the second method overcomes the problem that a sheet processed with the first method does not practically accept thereon further writing with a ball-point pen or an ink or attachment of a label. However, the second method is still disadvantageous in that, because a release agent is present within a fixed image, light scattering results, which leads to degradations in transparency of an OHP film and color producing property.
A wet fabrication method for manufacturing a xerographic toner includes a heating step in which heat is applied to various materials of the toner particles (for example, in an emulsion polymerization aggregation method, this step corresponds to the fuse or coalesce step; in a suspension polymerization method, the polymerizing monomer polymerization step). Typically, after the heating step, the toner is cooled, then the surfaces of the toner particles are processed with an acid or base to provide charging property, fluidity, and storage stability to the particles. A problem specific to the wet fabrication method is that, during the cooling, the release agent precipitates onto the toner surface due to the volume reduction caused by a difference between the glass transition points (Tg) and/or melting points of the binder and the release agent.
In general, the volume of a material decreases when the material changes from liquid to solid phase, and volume of the solid material (without change of phase) decreases as the temperature is lowered. In a particle composed of a single component, such a volume reduction would occur uniformly within the overall particle. However, in a toner particle including a binder having a release agent mixed therein, when both the binder and the release agent have low crystallinity, the binder having Tg and melting point higher than those of the release agent would contract at a higher rate at the same temperature compared to the release agent. Due to this contraction stress of the binder, the release agent would precipitate from where the binder pressure becomes low. This disadvantageous phenomenon would occur more noticeably in small-particle toners having a smaller volume to surface area ratio.
Typically, an external additive is applied to the toner surface so as to provide charging property, fluidity, and transferability to the toner surface. Because the release agent has a hardness less than that of the binder, if the release agent is precipitated onto the toner surface, the external additive would become easily buried within the precipitated release agent upon application of a physical shock along with heat. As a result, the charging property of the toner particles may become unstable, and degradations in fluidity and transferability may be caused. Further, undesirable adhesion of the toner to device parts may occur, and, when a two-component developer is used, undesirable adhesion of the toner to carrier may occur.
In order to prevent precipitation of the release agent onto the toner surface, various measures have been devised. More specifically, attempts for that purpose have been made to reduce the amount of the added release agent, reduce the domain size of the release agent within the toner, and increase the thickness of the binder layer located on the surface of the toner.
For example, Japanese Patent Laid-Open Publication No. Hei 10-207116 discloses the control of the amount of release agent within the toner and on the toner surface. Further, this publication recites that, by controlling the dispersion diameter of the release agent within the toner, a toner which allows to create high-quality color images and OHP images having excellent transparency can be produced.
Although the above-noted measures are effective in preventing precipitation of the release agent onto the toner surface, those measures also have a negative influence on migration of the release agent to the fixed image surface during fixation and disadvantageously reduce glossiness of the fixed image surface. Furthermore, the separating ability of the heat roll surface from the developer may become degraded, which would cause a hot offset. As such, there exists a requirement for a new small-particle toner having controlled surface structure.
Further, according to the method described in Japanese Patent Laid-Open Publication No. Hei 10-207116, when the toner fabrication process includes a fuse or coalesce step using heat, a slow cooling step performed after the heating causes crystallization of the release agent. Accordingly, if this method is employed to produce a small-particle toner, the particle size cannot be controlled.
The present invention relates to a small-particle toner including a release agent, and provides an electrostatic latent image developing toner having stable charging property, fluidity, and storage stability, which allows to create a high-quality image having high transparency. The present invention also provides a method for manufacturing an electrostatic latent image developer and electrostatic latent image developing toner having such characteristics.